A chlorine agent such as sodium hypochlorite is added, for example, to daily life water such as tap water, stored water and pool water, and the like. Addition of such a chlorine agent is aimed at the effects such as sterilization and disinfection by its oxidative action. However, if there are suspended solids, organic matters, metal ions, or the like in the water, the chlorine agent reacts with these substances and diffuses into air in open systems such as water storage tanks and pools. It is thus needed for preservation of the effects to measure the residual chlorine concentration in the water to which the chlorine agent is added periodically and thus to assure that the chlorine agent is kept at a desired concentration.
In water treatment systems employing various filtration membranes such as microfiltration membrane, ultrafiltration membrane, and reverse osmosis membrane, the filtration membranes are easily degraded by oxidation if the chlorine agent is oxidized in the water to be treated. As oxidation of the filtration membrane leads to deterioration in water quality of the treated water, normally in such a water treatment system, an activated carbon filter apparatus or a sodium bisulfite-adding apparatus is installed upstream of the filtration membrane for removal of the chlorine agent from the water to be treated. It is needed then to monitor periodically the residual chlorine concentration in the water discharged from the activated carbon filter apparatus or the water to be treated containing added sodium bisulfite and examine whether the chlorine agent is decreased to a concentration without any problem or removed securely.
In the measurements for monitoring chlorine concentration in water described above, a color reagent that develops color in reaction with chlorine in water is used. Examples of the color reagents known include N,N-diethylphenylenediamine (DPD), benzidine compounds, and the like (see, for example, Patent Documents 1 to 3).
Claim 1 of Patent Document 1 describes “a chlorine-concentration-measuring composition, comprising the following components: (a) a particular benzidine compound represented by Formula (I) or the salt thereof and (b) a composition containing one or more substances selected from the group consisting of chelates and surfactants.”
Claim 1 of Patent Document 2 describes “a composition for measuring the concentration of residual chlorine contained in water to be measured, comprising an aqueous solution containing a particular benzidine compound represented by Formula (I) and a surfactant, wherein the content of the surfactant is set to 0.5 mass % or more and 0.5 to 2.5 times by weight larger than that of the dialkylbenzidine compound and the pH is adjusted in an acidic range.”
Claim 3 of Patent Document 3 describes “a method for determining residual chlorine amount, characterized by examining the residual chlorine amount in a test solution based on the color tone of the colorant generated when a solution containing a tetraalkylbenzidine or the salt thereof is mixed with the test solution.”
The residual chlorine above is the sum of free chlorine (free residual chlorine) and bound chlorine (bound residual chlorine) and also referred to as total chlorine.